Proof of Kraft-McMillan

Question

In a recently class, my professor presented a proof of McMillan's inequality. I've been looking at his proof, but for some reason my brain is not making the necessary connection for the step shown below.

Where does the part on the right come from?

Answer 1

Write $(\sum_i x_i)^n = (\sum_i x_i)\cdots (\sum_i x_i) = (\sum_{i_1} x_{i_1}) \cdots (\sum_{i_n} x_{i_n}) = \sum_{i_1} \cdots \sum_{i_n} x_{i_1} \cdots x_{i_n}$.

Then let $x_i = 2^{-l_i}$.